The present invention relates generally to a system for drying grain stored within a conventional hopper, bin or the like. More particularly, the present invention is directed to methods and apparatus for drying grain by circulating air and adding controlled amounts of heat energy in response to critical sensed parameters.
In the past a wide variety of grain drying systems have been proposed. During the harvest, as will be readily appreciated by those skilled in the art, it is desirable and necessary to temporarily store grain. Harvested grains such as rice, wheat, and the like usually must be dried at least to a certain extent in order to provide an acceptable marketable commodity. Moreover it will be appreciated that the failure of the farmer to monitor the dryness of the grain often leads to reduced product quality.
Rice, for example, is most marketable when its inherent moisture content is approximately thirteen percent (13%). Harvested rice, depending upon field conditions and obviously variable weather conditions, may be characterized by a moisture content of twenty percent or more. Usually rice is not harvested until its moisture is twenty four percent (24%) or less. When rice is too dry (i.e. less than approximately thirteen percent moisture) its value for subsequent processing will be reduced. The problems inherent in storing overly moist grain are well known. For example, when stored for long periods of time overly wet grain may be subjected to souring, mold growths, rotting and the like. Consequently numerous prior art solutions have been utilized for drying grains.
Stored grain is known to exhibit stable long term moisture characteristics which are a result of the balance of the partial pressure of the moisture in the grain with the partial pressure of the moisture in the surrounding air. One of the most common ways to express this relationship is to compare grain equilibrium moisture with the relative humidity of the drying air. Temperature is also obviously important, but to a lesser extent. The most important parameter in determining relative humidity is depression, or the difference between wet bulb and dry bulb temperatures. Hence grain will be continually exchanging moisture to remain in equilibrium with the specific air which contacts it and vice-versa.
Typical grain storage bins include an internal, usually perforated floor upon which stored grain is supported. A plurality of perforations or vent passageways defined in the floor admit air through a lower plenum from associated drying apparatus to circulate air through the bin and hence dry the stored commodity. An associated fan mounted in a suitable housing externally of the bin drives air into the bin plenum through suitable ducts, and usually gas fuels an associated heater for selectively heating the air stream. Usually the air temperature is determined by "on-off" type servocontrols which may be set to regulate temperature and/or relative humidity in a continuously cycling fashion. Usually some form of stirring apparatus associated with the bin is employed during drying for circulating the grain mass. As air circulates through stored, preferably agitated grain, the net result is that some of the grain will be dried and as the cycle continues some will absorb moisture. In other words, as the conventional drying process continues various portions of the stored commodity will first lose and then gain moisture. The latter disadvantageous phenomena is related to the grain equilibrium moisture content and the quality of the circulating air.
Conventional drying systems include a gas driven heater typically capable of adding heat energy to the drying air stream at the rate of between Seven Hundred and Fifty Thousand to over One Million BTU's per hour, which is enough to raise the flowing air temperature by fifty to eighty degrees fahrenheit. The various controls employed with such systems may include temperature switches, humidistats, which are devices responsive to relative humidity for opening or closing circuits, as well as thermocouples and the like. The aforementioned control devices affect fuel flow. Often prior art systems suffer from swings between heat and no heat conditions due to their mechanical nature, and to the time required to constantly dilute drying air near the outer wall of the bin plenum where various sensors or controls are usually mounted. This often results in a temporary period of high temperature rise which heats and overdrys grain, followed by a period where only unconditioned night air is circulated which cools and rewets the stored grain. As a result the commodity is subject to temperature and moisture induced shock.
It is also well known that when the temperature of the drying air is increased the relative humidity of the air will decrease, and hence its drying potential over a limited period of time increases. As has previously been recognized, the temperatures to which the grain may be subjected during drying are critical. Large, rapidly occurring temperature and/or moisture swings can cause fragile rice kernels, for example, to crack and break apart resulting in lowered crop quality or value. Deleterious effects from such shocks are particularly significant for the rice or grain disposed at the bottom of the bin. Consequently stirring apparatus is critical with conventional drying systems. Essentially stirring is mandatory to prevent destruction of grain near the bin bottom.
Much of the heating equipment for grain drying has been developed for crops other than rice and does not properly adapt to the specific application of rice drying. Newly harvested rice often contains a mold which is active at temperatures above eighty degrees fahrenheit and at a moisture content above approximately sixteen percent. The activity of this mold is exothermic so that as the mold grows it generates more heat, and the generated heat encourages greater mold growth. Hence one common practice has been to add heat to the forced air thereby increasing its drying potential to quickly remove moisture content above fifteen percent in the blended grain. Another approach has been to run the fan continuously for months without heating to keep the grain relatively cool and minimize mold activity.
One major problem with prior art devices is that the typical apparatus employed, such as the hardware previously developed for drying corn, will raise rice temperatures much more than what is necessary to dry the crop to approximately thirteen percent moisture. As suggested previously, because of the hardware typically employed with prior art corn drying systems rice within lower levels of the bin experiences rapidly cycling, relatively high temperatures, and the milling quality of such mistreated rice usually suffers.
The drying relationships of air and rice are based upon a wide variety of complex thermodynamic properties. In order to simplify the drying hardware, and concomitantly to decrease its cost while increasing its reliability, experiential data must be simplified in an effort to provide a concrete design goal. A variety of previously published data exists to determine the general values for equilibrium moisture content, which is the moisture content that grain will eventually arrive at if exposed to specific air conditions for a long enough period of time. Data suggested by or associated with the prior art does not reveal how fast stored grain will lose moisture as it approaches equilibrium. Instead, practical experience and observation provides general principles for postulating the long term parameters forming the goal of drying. Generally drying air characterized by a temperature in excess of the daytime high temperature has been used to increase the rate of moisture removal. This well known principle is responsible for the unfortunate "overheating" characteristic of prior art design. To minimize grain degradation prior art drying processes are usually stopped before grain can reach equilibrium with the drying air, and the resulting batch is blended to achieve an average moisture distribution.
By way of example, a typical grain storage bin having a diameter of twenty four feet may store rice at twenty-two percent moisture sixteen feet deep. Drying is effectuated by a conventional ten horsepower centrifugal fan which generates an airstream warmed by an associated one million BTU/hr. gas heater which attempts to heat the plenum to ninety degrees fahrenheit in a cycling fashion. Typically a harvest-time overnight low will be approximately sixty degrees, and the farmer will continuously operate his bin stirring apparatus. It would take such a system approximately ten days to dry the rice to the desired thirteen percent moisture (wet weight basis) if the initial "heat up" energy is recovered. In this example 5819 bushels are dried at a total drying cost of $569.56. The latter total includes approximately $166 for electrical fan power, $388 for gas (propane) fuel, and $15 for the stirring machine. An observed drying expense of approximately 9.8 cents per bushel resulted, and approximately 2079 BTU's of energy wave expended per pound of water removed.
In the prior art a wide variety of drying systems are known. Barre in U.S. Pat. No. 2,855,697 discloses a supplemental heat system for drying crops. The latter reference employs a humidistat disposed within the bin plenum for monitoring relative humidity therein. U.S. Pat. No. 3,217,424 additionally illustrates the use of a humidistat disposed below the bin plenum to turn on an associated heater whenever the humidity of air moving through the plenum chamber rises above a predetermined value. Similarly, Pfeiffer in U.S. Pat. No. 3,470,626 employs a plenum-mounted humidistat for initiating heater control. Relevant grain drying apparatus is also disclosed in U.S. Pat. Nos. 4,134,216; 3,934,355; 4,043,051; 2,716,289; 4,270,280; 2,606,372; and 3,264,118. U.S. Pat. No. 2,894,391 discloses a motor aspirated psychrometer which has been found extremely useful for measuring wet bulb temperature. Other relevant prior art includes abstract 46,214, seen on pages 985-986 of Volume 647 of the Official Gazette, 6/19/51.
A basic precept inherent in common prior art designs known to us implicitly assumes the necessity of providing heating in the order of one million (1,000,000) BTU's per hour. We have found that it is far more desirable to introduce less heat.
Moreover, we have found it desirable to limit heating input to between several thousand and approximately three hundred thousand BTU's per hour. Moreover our tests have verified that instantaneous moisture removal rates are reduced when conditioned air is generally warmer than the temperature of the rice and are increased when it is cooler. When the temperatures of the air and the grain are approximately equal, exit air quality is at equilibrium with the grain at the top of the bin, and a comparison of entrance and exit quality reveals adiabatic changes in state.
We have further determined that the observed wet bulb temperature for the majority of the grain producing regions of the United States, when measured in the evening or during the night, fall within one or two degrees of the observed overnight low. The wet bulb temperature is substantially constant during the night. This affords the opportunity to control drying air relative humidity by basing a control system upon the overnight low. As used herein the term "control depression" refers to the difference between plenum dry bulb temperature and wet bulb temperature at the overnight low. Classic depression is the single most important factor which determines relative humidity. A substantially constant relative humidity beneath the grain bin plenum may thus be maintained by preserving a constant depression. We have found it desirable to raise the temperature of drying air only an amount approximating the control depression. As will be appreciated by those skilled in the art, a reading of standard psychrometric reference charts will reveal the relationship between relative humidity, temperature and depression. Once the desired grain equilibrium moisture is referenced against known outside temperature in standard reference tables, for example, relative humidity of the required drying air may be ascertained. Subsequent reference to a standard psychrometric chart will reveal the depression required for successful practice of the present inventions.